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The force, specialist vets and Melbourne University are investigating what has caused the spate of illness.

The condition first emerged in December and several possible causes have already been ruled out.

"At this stage, we're told there was a number of common factors which we are looking at," squad Inspector Jon Woodyatt told 3AW Radio on Friday.

"One of them does include the food that we're feeding the dogs."

Nine dog squad members were affected by the illness. (AAP) (supplied)

It is "extremely rare and unheard of in veterinary circles" for a group of dogs from the same area to fall ill, a police statement read, especially across the force's two breeds, German shepherds and labradors.

Mars Petcare Australia will continue to provide information to the university and says concerned pet owners should contact their vet, or the company.

People who have bought ADVANCE Dermocare dry dog food in 3kg, 8kg and 15kg bags are being asked to return them to the place of purchase for a refund.

The products would have been bought from speciality retailers or vets, and no other ADVANCE products are affected by the recall, the company said.